1. Field of the Invention
The invention generally relates to the field of semiconductor processing. More particularly, the invention relates to monitoring wafer condition in a plasma processing system.
2. Background Information
The temperature of a substrate during plasma processing affects device quality and yield. For example, the etch process, e.g., etch selectivity, is dependent upon the wafer temperature. Wafer gas backflow cooling may be used to increase the amount of heat transferred from a wafer to a chuck.
The main chuck structure, for example, the chuck block, may be made of a material that has a high thermal conductivity which allows heat transferred from the wafer to be absorbed by a cooling system, which typically consists of a liquid coolant flow channel, a circuit with thermoelectric coolers, or a combination thereof. The high heat conductivity ensures that the chuck block maintains a relatively uniform temperature throughout its volume even at high plasma heat loads.
The wafer is typically held on an electrostatic chuck by an electrostatic attractive clamping force. The electrostatic attractive clamping force is exerted on the wafer by one or more chucking electrodes, embedded within the chuck, and biased to a high DC voltage. At the same time that the electrostatic attractive clamping force is exerted, a heat transfer gas, such as helium or argon, may be injected into the microscopic gap that remains between the chuck and wafer. The heat transfer gas is typically at pressures much higher than the process chamber pressure, to enhance the heat transfer between the wafer and chuck.